1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor memory device. The present invention particularly relates to a method of manufacturing a non-volatile semiconductor memory element having a limit on the number of times of rewriting information and a volatile semiconductor memory element such as DRAM having no upper limit on the number of times of rewriting information with high integration on the same semiconductor substrate and at low cost.
2. Description of Related Art
In recent years, resistance RAM (RRAM) having a variable resistance memory element as memory that is used in information equipment such as computers and mobile telephones and the like has been receiving attention. A variable resistance memory element is a memory element that is basically constituted by a thin film resistor such as a binary metal oxide, a transition metal oxide, an aluminum oxide, a silicon oxide or germanium oxide being sandwiched between two electrodes, and makes use of the phenomenon in which the resistance of the thin film resistor changes by impressing an electrical field or flowing an electrical current across the electrodes.
Since the resistance value of the thin-film resistor is stored even when the impression of the electrical field or current is removed, it is possible to realize non-volatile memory. The information that is stored in the variable resistance memory element can be read out by comparing the resistance value with a reference value. This kind of memory element is generally called RRAM as mentioned above. It is possible to constitute a variable resistance memory element in an array such as DRAM (Dynamic Random Access Memory) by combining with a selection transistor.
For example, FIG. 25 shows a non-volatile semiconductor memory element disclosed in Japanese Unexamined Patent Application, First Publication, No. 2004-342843 as an example of a variable resistance memory element that can readily store and read out information, and can be readily manufactured with a comparatively easy manufacturing method. FIG. 26 shows a non-volatile semiconductor memory element disclosed in Japanese Unexamined Patent Application, First Publication, No. 2006-173267 as an example of another example of a variable resistance memory element that can stably perform operations such as recording information with a constitution that is stable even with respect to heat.
The non-volatile semiconductor memory element shown in FIG. 25 has a substrate 401, a lower electrode 402, an insulation film 403, an amorphous thin film 404, an upper electrode 405, an electrode layer 406, and a conduction layer 407. Also, non-volatile semiconductor memory element shown in FIG. 26 has a substrate 501, a lower electrode 502, an ion source layer 503, a memory layer 504, an insulating layer 505 denoted by reference numeral 505, and an upper electrode 506.
The conventional memory elements shown above utilize the oxidation-reduction action of metal, such as silver, copper, or zinc. Specifically, when an electrical field is impressed on a thin-film resistor (the amorphous thin film 404 and the memory layer 504) consisting of a transition metal oxide, an aluminum oxide, a silicon oxide or germanium oxide sandwiched between two electrodes (the upper electrodes 405 and 506 and the lower electrodes 402 and 502), the aforementioned metal that exists in one electrode is oxidized (ionized), moves through the thin-film electrode to be reduced and deposited at the other electrode, whereby the resistance value of the thin-film electrode falls. Also, when an electrical field is impressed in the reverse direction, the deposited metal is once again ionized and returns to the original electrode, and so the resistance value of the thin-film electrode rises. Thereafter, even if the electrical field between the electrodes is removed, the resistance value of the thin-film resistor is maintained, and information is stored in a non-volatile manner using this phenomenon. In addition, these memory elements are capable of reading and writing at high speeds on par with DRAM, and so their use as non-volatile RAM has been expected.
However, it is has been reported that the abovementioned memory elements have a limit to the number of changes of the resistance value of the thin-film resistor, that is, the number of times of rewriting information. Accordingly, the abovementioned memory element is not suitable for uses in which information is frequently rewritten as work memory, such as DRAM (Dynamic Random Access Memory) Therefore, there is a problem that it cannot be used as non-volatile RAM for realizing computer systems, mobile phones, information appliance products, etc. with an instant-on function.
On the other hand, while DRAM is suited to uses such as information being frequently rewritten as work memory, due to its being volatile memory, it has the characteristic of the information being lost when the power is turned off.
In this way, since RRAM and DRAM have a relationship of complementing each other, it has been desired for them to be used in combination. Therefore, if it were possible to form these two types of memory elements with the same manufacturing process and on the same chip, it would be possible to provide a memory system that is easy to use at a low cost, and so its realization has been sought.
The present invention has been achieved in view of the above circumstances, and has as its object to provide a method of manufacturing a non-volatile semiconductor memory having a limit on the number of times of rewriting information and a volatile semiconductor memory element such as DRAM having no upper limit on the number of times of rewriting information with high integration on the same semiconductor substrate and at low cost.